Well, perhaps you could explain the situation to 
the seller and see if he/she would be willing to 
set up a payment plan.

Still, even though I have no idea what this cel 
may be, I have to wonder if it's worth $600.  If 
it's *your* all-time dream cel, then the chances 
are extremely high that it's someone else's dream 
cel too.  Since some other collector didn't 
already snatch it up, I'd have to guess that this 
cel is extremely overpriced.  If it isn't 
overpriced, then it's already been sold and your 
problem is resolved.